1. Field of the Invention
The present invention relates to a device and method for determining usage of diluted versus saturated color inks in an inkjet printer. More particularly, the present invention relates to a device and method for determining the amount of diluted and saturated inks used for each color throughout the color range of a color inkjet printer.
2. Description of the Related Art
In recent years color inkjet printers have been developed for home and office use. These printers have typically used four different inks in the colors of cyan, magenta, yellow and black (hereinafter "CMYK") color inks. These color inks were of a single high concentration and were useful in generating reports, charts and graphs. However, due to the granularity of these saturated inks, it was not possible to create near photographic quality images.
To overcome this granularity problem in the saturated inks, diluted inks have been employed. These diluted inks are used to reproduce the less intense colors of the CIELAB system while the more intense colors require the usage of the saturated inks. Typically only diluted forms of the cyan and magenta are used in these printers and a total of six different inks comprising cyan, magenta, yellow, black, diluted cyan and diluted magenta (hereinafter "CMYKcm") are thus used.
The CIELAB system mentioned above is a color space to be used for the specification of color differences. It consists of three variables (L*, a*, and b*) as Cartesian coordinates to form a three-dimensional color space. The L* variable indicates perceived color lightness ranging from 0.0 for black to 100.0 for a diffuse white. The a* and b* dimensions correspond to the red-green and yellow-blue chroma perceptions, respectively. The a* and b* values can take on both negative and positive values. Their maximum values are limited by the physical properties of materials involved, including the inks themselves and the print medium used. When mixing the diluted and saturated inks, the L* dimension value of the CIELAB system is found more significant in determining graininess and color perception. If the L* has a smooth transition in perceived color and lightness in the mixing process, the other two dimensions (a* and b*) will also be perceived to be reasonably smooth.
In the printing method the printer or printer driver receives a red, green, and blue (hereinafter "RGB") digital color image and it is necessary to then convert this image to the appropriate CMYKcm values. To convert the RGB image to printed CMYKcm images; a three to six dimensional lookup table (not shown) is often used. This method gives more flexibility for color reproductions but slows the printing process since more online computations (interpolations) are necessary to determine all points in the color space. Therefore, the printing process takes considerably longer.
Further, the method of determining when and how much diluted ink to use versus saturated ink has been a subjective process. A technician would view wedges for each color using either the diluted or saturated ink or a mixture of the two and determine which the technician believed produced the best results for each color intensity.
Several problems are encountered using this subjective approach. First, the results vary according to the technician. Second, at some point the technician will have to switch from use of the diluted ink to the saturated inks. At this point, if the change is not done gradually, a sudden change in granularity and color lightness (L* value) will be detectable by the user. FIG. 1 shows an example of non-smoothed mixing of the diluted and saturated inks in the prior art. In this example, the diluted ink digital count first increases linearly from 0 to 255 (maximum) and then linearly decreases from 255 to 0. The saturated ink digital count is zero before the diluted ink reaches the maximum (255) and then linearly increases from 0 to 255 as shown in FIG. 1. This approach will generate a non-smoothed color lightness, L* value profile, as shown in FIG. 2.
As can be seen in FIG. 2, there is an abrupt change in color lightness value, L* value, when the saturated ink is introduced in the mixture. Further, the user sees a similar and even more dramatic perceptible abrupt difference in image granularity. This results in significant variations in a single given image in transitions from lighter to darker colors and prevents the achievement of near photographic image quality.
In order to overcome these problems a device and method is needed that systematically determines which ink concentrations to use for each color value. Further, the resolution of the image should be kept as high as possible by minimizing image graininess (granularity). Also, the processing required by the printer or the printer driver in determining which inks to use at the time of printing should be kept at an absolute minimum so that printing may proceed at the fastest possible rate. Finally, the device and method should provide for a smooth transition in granularity and color lightness from lighter to darker colors.